Acestridium Haseman, 1911

Delapieve, Maria Laura S., A., Pablo Lehmann & Reis, Roberto E., 2017, An appraisal of the phylogenetic relationships of Hypoptopomatini cascudinhos with description of two new genera and three new species (Siluriformes: Loricariidae), Neotropical Ichthyology 15 (4), No. e 170079, pp. 1-38 : 21

Genus Acestridium Haseman, 1911

Acestridium Haseman, 1911: 319 .

Type-species: Acestridium discus Haseman, 1911 . Type by monotypy. Gender neuter.

Included species. Acestridium colombiensis Retzer, 2005 ; Acestridium dichromum Retzer, Nico, Provenzano, 1999 ; Acestridium discus Haseman, 1911 ; Acestridium gymnogaster Reis, Lehmann, 2009 ; Acestridium martini Retzer, Nico, Provenzano, 1999 ; Acestridium scutatum Reis, Lehmann, 2009 ; Acestridium triplax Rodriguez, Reis, 2007 .

Diagnosis. Acestridium is diagnosed as monophyletic based on ten exclusive synapomorphies: large exposure of mesethmoid on dorsal surface of snout (char. 5.2); anterolateral margin of anterohyal concave (char. 32.1); lack of articulation of neural spine of sixth vertebral centrum to parieto-supraoccipital (char. 39.1); dorsal fin attached to neural spine of vertebral centrum 11-13 (char. 45.2); anterior margin of cleithrum convex, with hollow in mesial lamina (char. 58.1); lateropterygium absent (char. 61.1); ventral margin of infraorbital 4 expanded anteroventrally, its deepest point surpassing anterior margin of bone (char. 72.2); mid-ventral series of lateral plates absent (char. 83.3); and spatulate projections of snout are present (char. 90.1). In addition, there are 14 non-exclusive synapomorphies: ventral condyle of mesethmoid oval (char. 3.3); nasal capsule completely encapsulated (char. 7.0); posterolateral portion of lateral ethmoid narrower or as wide as anterior margin (char. 10.0); dorsal wall of swimbladder formed by compound pterotic and supraoccipital (char. 13.1); condyle of hyomandibula articulates to neurocranium only by prootic (char. 15.3); crest for insertion of levator arcus palatini muscle robust (char. 18.0); one lateral foramen in hyomandibula (char. 20.1); metapterygoyde-hyomandibula suture large (char. 21.0); interhyal absent (char. 33.1); second and third radials of pectoral fin joined along longest axis (char. 54.1); two postrostral plates (char. 66.2); odontodes on dorsal surface of pectoral-fin spine randomly distributed (char. 95.0); iris operculum present (char. 100.0); and lack of skin flap on dorsal surface of first pelvic-fin ray in males (char. 103.1).

Comparisons. The species of Acestridium are further distinguished from other hypoptopomatins by a highly distinctive slender and elongated body, usually with green color in life. Acestridium is further distinguished by a snout that is produced into an anterior spatulate projection that bears hypertrofied odontodes ( vs. snout not elongated) and by lacking the mid-ventral series of lateral plates ( vs. midventral series present). They also have very delicate and poorly developed fins and the odontodes of the trunk are small and aligned forming conspicuous rows ( vs. normally developed fins and odontodes not clearly aligned). It differs from all other genera, except for some species of Otocinclus , by possessing an iris operculum ( vs. iris operculum absent). Acestridium is additionally distinguished from Hypoptopoma , Nannoptopoma , Otocinclus , Leptotocinclus , and Nannoxyropsis by the caudal peduncle, which is strongly depressed, elongated, and narrow ( vs. caudal peduncle oval in cross-section). Acestridium is further distinguished from Niobichthys , its sister genus, by having a spatulate snout projection ( vs. spatulate snout projection absent), by ten or 12 branched rays in the caudal-fin ( vs. 14 branched rays); by the lateral-line canal that shifts to the ventral series of lateral plates after truncation of the median plate series ( vs. shifting to the dorsal series); and by odontodes on the dorsal surface of the pectoral-fin spine that are randomly arranged ( vs. arranged in two or three rows).